Update from the Field: Who is the Weed’s Daddy?
Posted by Dave Dyson, Agronomist on September 25, 2018
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In the coming weeks, combines will be rolling and this year’s soybean crop will be coming off. As my travels take me across the Corn Belt, I have noticed areas in mature soybean fields that have been taken over with weeds. These weedy areas can suppress the yield, harbor insects, and provide a huge weed seed bank for next year. Early reports show a monstrous crop out there, but before we cut our soybeans, we need to look at the ending weed pressure that’s out in these fields.
There can be several reasons certain weeds have not been controlled throughout the growing season. One of the most common reasons I see, year after year, is not using proper rates of herbicides. There can be multiple reasons why an applicator would not use the correct rate of herbicides on weeds growing out in their fields. Weather can be a big factor, as weeds can grow at an extraordinary rate during hot/wet conditions. It is not enough to simply spray the prescribed rate for a previously-scouted field if there was a delay due to weather or mechanical breakdown. The field should be scouted again, re-evaluated for any new growth, and herbicide rates should be adjusted to compensate for the new growth.
The cost of herbicides can be a factor as well. Some growers have reduced the amount they apply to “cheapen up” their herbicide application. The herbicides are labeled for certain rates according to the height of the weeds. Reducing these rates will only lead to multiple applications or weed populations that become resistant to the herbicides.
Insects can also help weeds escape a death-by-herbicide-application scenario. Some stalk-boring insects may hollow out the inside of weeds (figure 1), making translocating herbicides from the leaves to the roots almost impossible. Killing weeds earlier, before they get big enough to be an attractive home to boring insects, should be forefront on growers’ minds next spring. Start with an application this fall of a crop-specific herbicide. Following with an application of a good residual herbicide next spring will help to ensure weeds will not get out of control, even if weather throws a wrench into early-post spraying.
Figure 1: This picture shows a European corn borer hollowing out a giant rag weed, disrupting the herbicide from translocating to the roots. This picture was taken near Kokomo, IN in 2018.
Some fields are still infested with weeds that are not being killed with this year’s herbicide applications. Those weeds will produce offspring, and we need to know if next year’s weeds will be resistant to a certain mode of action. Similar to the objectives of some daytime talk shows, we need to understand the paternal identity of next year’s weeds. If we can determine what this year’s weeds are resistant to, we can make informed decisions on which herbicide to apply next year. Both Purdue University and The University of Wisconsin – Madison have developed processes to help you determine what kind of resistance you may have out in the field. The following links will guide you to their submittal forms:
In conclusion, information can be a very powerful tool. Knowing what mode of action your weeds are resistant to can help with next year’s seed and herbicide buying decisions. If you need help taking samples or advice on buying seed and herbicides for next year, you can always contact me or one of the Agronomy Advisors from The Andersons.
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Dave Dyson is a regional agronomist for The Andersons’ Farm Centers which are located throughout Ohio, Michigan, and Indiana. He is an Indiana native and grew up on a dairy farm in Miami County. A graduate of Purdue University with a degree in Crop & Soil Science, Dave has a deep knowledge of various agronomic topics and is committed to helping growers improve their crops. If you have any questions, Dave can be reached at david_dyson@andersonsinc.com.
